This invention relates to contact image sensors in general and more specifically to a contact image sensor assembly having optical detectors and light emitting diodes mounted on a single circuit board with a light guide positioned under the light emitting diodes.
Contact image sensors are electronic optical detectors commonly used in devices such as computer scanners, fax machines, and digital copiers. A contact image sensor is an optical sensor assembly which is placed on or near an object during a scanning operation. The contact image sensor produces machine-readable image data signals that are representative of the scanned object, such as a photograph or a page of printed text. In a typical contact image sensor application, the image data signals produced by the sensor may be used by a computer processor to reproduce an image of the scanned object on a suitable display device, such as a CRT or a printer.
A typical contact image sensor may include an elongate optical assembly comprising illumination, optical imaging, and detection systems. The illumination source illuminates a portion of the object (commonly referred to as a xe2x80x9cscan regionxe2x80x9d), whereas the optical imaging system collects light reflected by the illuminated scan region and focuses a small area of the illuminated scan region (commonly referred to as a xe2x80x9cscan linexe2x80x9d) onto the surface of a photosensitive detector positioned within the contact image sensor. The photosensitive detector converts the image light incident thereon into electrical signals representative of the scan line. Image data representative of the entire object then may be obtained by sweeping the scan line across the entire object.
The term xe2x80x9cimage lightxe2x80x9d as used herein refers to the light reflected from the document and focused onto the surface of the detector array by the optical system. The image light may be converted into digital signals in essentially three steps. First, the photosensitive optical detector converts the light it receives into a varying electric current. Second, the varying electric currents from the detector elements are converted into analog voltages by an analog amplifier. Finally, the analog voltages are digitized by an analog-to-digital (A/D) converter. The digital data then may be processed and/or stored as desired.
While contact image sensors of the type described above are being used in many applications, they are not without their problems, such as bulky illumination sources and uneven illumination across the scan line. For best scanning results, the object must be illuminated by the illumination source at an angle to reduce or eliminate specular reflections. To do this, the photosensitive detector is typically mounted on one circuit board, and the illumination source is placed on another circuit board. The circuit board with the illumination source is then angled at about a 45 degree angle with respect to the object. This additional circuit board increases size, cost, and difficulty of assembly of the contact image sensor. This also results in poor depth of illumination. For example, if the contact image sensor is moved even a small distance from the object, the illumination level drops off rapidly. Even if the optical system depth of field is good and the object remains in focus as the contact image sensor is lifted, the poor depth of illumination will cause the image of the object to appear too dark or too unevenly illuminated.
In addition, the illumination source typically consists of a linear array of light emitting diodes (LEDs), with gaps between each LED. The illumination across the scan line is therefore not very uniform, having bright areas under the LEDs and dimmer areas under the gaps. This lack of uniformity of illumination can be compensated for somewhat by amplifying the signal from the dimmer areas, either in the detection system hardware or with software. However, this slows the image processing software and increases the noise in the amplified signal, resulting in poorer image quality.
The LEDs in the illumination source are typically designed to radiate light over a very wide angle, such as 180 degrees. Only a portion of this light is directed at the scan line, resulting in an inefficient use of the illumination from the LEDs.
Finally, placing the illumination source on a separate circuit board increases the width of the contact image sensor and gives a poor cosmetic appearance to an otherwise high tech product.
Consequently, a need exists for a contact image sensor with more integrated packaging of the illumination source and detection system, resulting in a more compact unit which is simpler to assemble and align. A further need exists for a contact image sensor having improved depth of illumination. A further need exists for a contact image sensor which produces more uniform illumination across a scan line. A further need exists for a contact image sensor which efficiently uses the illumination from the illumination source.
To assist in achieving the aforementioned needs, the inventors have devised a contact imaging sensor with a light guide. The light guide collects the light from a light source and directs it onto a scan line region. The light guide also increases the uniformity of illumination across the scan line region and increases the depth of illumination.
The invention may comprise a contact image sensor which includes a light sensitive optical detector and a light source mounted on a mounting surface. A light guide is located under the light source and is oriented to direct a light path from the light source to a scan line region under the light sensitive optical detector.
The invention may also comprise a method of producing a contact image sensor. The method comprises mounting a light sensitive optical detector on a mounting surface and mounting at least one light source adjacent the light sensitive optical detector on the mounting surface. The method further comprises positioning a light guide under the at least one light source to direct a light path from the at least one light source to a region under the light sensitive optical detector.
The invention may also comprise a contact image sensor having a printed circuit board with a plurality of photodetectors and a plurality of light sources mounted to the printed circuit board. The contact image sensor also comprises means for directing light from the plurality of light sources to a region under the plurality of photodetectors.